The objectives of the proposed research are to evaluate intrarenal metabolism of sodium in normal animals and in animals with chronic sodium retention. The evaluation will involve micropuncture techniques and isotope tracer studies to: 1) characterize sodium, calcium and phosphate reabsorption along the various nephron segments; 2) determine the relationship between sodium reabsorption and the reabsorption of calcium and phosphate along the various nephron segments. These studies will include control animals, animals with mineralocorticoid induced sodium retention, and following acute increases in the delivery of fluid from the proximal tubule. BIBLIOGRAPHIC REFERENCES: Schneider, E.D. Effects of parathyroid hormone secretion on sodium reabsorption by the proximal tubule. Am. J. Physiol. 229(5): 1170-1173, 1975. Schneider, E.G., and A.L. McLane. Renal capillary microinjection of 32PO4 3H-inulin during hydropenia and extracellular volume expansion in the dog. Fed. Proc. 35: 908, 1976.